Conventional Doppler signal frequency analysis is carried out with both of a frequency slot width and a time slot width being fixed. Since the frequency and time slot widths determine frequency and time resolutions, respectively, of the frequency analysis, these frequency and time resolutions are also fixed in the conventional frequency analysis. The frequency and time slot widths are reciprocals of each other so that if one of these slot widths is reduced, then the other slot width is increased, and so are the frequency and time resolutions.
Doppler signal frequency analysis serves to analyze a Doppler signal obtained over a time period corresponding to the time slot width. Therefore, a delay is caused over the time corresponding to the time slot width before analyzed results are produced. For example, if the frequency slot width is 50 Hz and the time slot width is 20 msec. as with a typical example in a medical ultrasonic Doppler system, then the delay time is 20 msec. The time resolution is also 20 msec. since the time slot width is the same as the time resolution. Where the ultrasonic Doppler system is employed for heart diagnosis, the results of frequency analysis of the Doppler signal are displayed or recorded simultaneously with an M-mode ultrasonic image and an electrocardiogram or a phonocardiogram for mutual comparison. Although the M-mode ultrasonic image and the electrocardiogram or phonocardiogram are produced without any substantial time delay and with a high time resolution, the results of Doppler signal frequency analysis are obtained with a time delay of 20 msec., for example, and a coarse time resolution. Therefore, it is not suitable for such analyzed results to be used for comparison with the M-mode ultrasonic image and the electrocardiogram.
In order to reduce the time delay and increase the time resolution, the time slot width should be reduced. However, when the time slot width is reduced, the frequency slot width is to be increased, resulting in a lowered frequency resolution. As a result, the accuracy of frequency analysis becomes lower in a lower frequency band.
The fixed frequency and time slot widths cannot therefore meet contradictory requirements for the frequency resolution and the time resolution or the time delay.